Method and device for determining a level of ventilatory assist to a patient

ABSTRACT

A method and device determine a level of mechanical ventilatory assist to be delivered to a patient. A patient&#39;s neural respiratory drive is measured and multiplied by an existing amplification factor to obtain an existing predicted ventilatory pressure. A mechanical ventilator is controlled accordingly. An existing resulting pressure is measured. The patient&#39;s neural respiratory drive is multiplied by a modified amplification factor to obtain a new predicted pressure. The existing and new predicted pressures are compared to determine an anticipated change in pressure. The mechanical ventilator is now controlled according to the new predicted pressure. A new resulting pressure is measured. The existing and new resulting pressures are compared to determine an actual change in pressure. The anticipated and actual changes in pressure are compared. The amplification factor is increased, maintained or decreased in response to the comparison between the anticipated and actual changes in pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Application No. 61/193,987, filed Jan. 15, 2009, and is a U.S. national phase of PCT International Application No. PCT/CA2010/000043, filed on Jan. 14, 2010, both applications are incorporated herein by reference.

FIELD

The present invention is concerned with determining a level of ventilatory assist to be delivered a patient.

BACKGROUND

In practice, an adequate level of ventilatory assist to be delivered to a patient under mechanical ventilation is difficult to determine since unloading of the patient involves compensation for increased respiratory demand in terms of metabolic drive, resistive and elastic loads imposed by the patient's respiratory system, as well as weakness of the inspiratory muscles. There is therefore a need for a technique that facilitates such determination.

SUMMARY

The present invention relates to a method for determining a level of ventilatory assist to be delivered to a patient by a mechanical ventilator in response to a measure of a patient's neural respiratory drive multiplied by an amplification factor, comprising: calculating an existing predicted ventilatory assist pressure; measuring an existing resulting pressure delivered to the patient by the mechanical ventilator; changing the amplification factor from an existing amplification factor to a new amplification factor; calculating a new predicted ventilatory assist pressure using the new amplification factor; measuring a new resulting pressure delivered to the patient by the mechanical ventilator after the amplification factor has been changed from the existing amplification factor to the new amplification factor; comparing the new predicted ventilatory assist pressure and the existing predicted ventilatory assist pressure to determine an anticipated change in pressure that will be delivered to the patient by the mechanical ventilator; comparing the new resulting pressure and the existing resulting pressure to determine an actual change in pressure delivered to the patient by the mechanical ventilator; comparing the anticipated change in pressure with the actual change in pressure; and delivering a decision to increase, maintain or decrease the amplification factor in response to the comparison between the anticipated change and the actual change in pressure.

The present invention is also concerned with a device for determining a level of ventilatory assist to be delivered to a patient by a mechanical ventilator in response to a measure of a patient's neural respiratory drive multiplied by an amplification factor, comprising: a first calculator of an existing predicted ventilatory assist pressure; a first sensor of an existing resulting pressure delivered to the patient by the mechanical ventilator; a modifier of the amplification factor from an existing amplification factor to a new amplification factor; a second calculator of a new predicted ventilatory assist pressure using the new amplification factor; a second sensor of a new resulting pressure delivered to the patient by the mechanical ventilator after the amplification factor has been changed from the existing amplification factor to the new amplification factor; a first comparator of the new predicted ventilatory assist pressure and the existing predicted ventilatory assist pressure to determine an anticipated change in pressure that will be delivered to the patient by the mechanical ventilator; a second comparator of the new resulting pressure and the existing resulting pressure to determine an actual change in pressure delivered to the patient by the mechanical ventilator; a third comparator of the anticipated change in pressure and the actual change in pressure; and a third calculator of a decision to increase, maintain or decrease the amplification factor in response to the comparison between the anticipated change and the actual change in pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the method and device for determining a level of ventilatory assist will become more apparent from reading of the following non restrictive description of illustrative embodiments thereof, given by way of example only with reference to the accompanying drawings in which:

FIG. 1 is a schematic flow chart of a method for determining the level of ventilatory assist to be delivered to the patient by the mechanical ventilator in response to the measure of the patient's neural respiratory drive multiplied by the amplification factor; and

FIG. 2 is a schematic block diagram of a device for determining the level of ventilatory assist to be delivered to the patient by the mechanical ventilator in response to the measure of the patient's neural respiratory drive multiplied by the amplification factor.

DETAILED DESCRIPTION

The method and device for determining the level of ventilatory assist to be delivered to the patient by the mechanical ventilator in response to the measure of the patient's neural respiratory drive multiplied by the amplification factor will now be described.

The level of ventilatory assist to the patient may be controlled by applying to the mechanical ventilator a control signal representative of a predicted ventilatory assist pressure P_(target) determined as a function of the patient's neural respiratory drive and an amplification factor AF. For example, the electrical activity of the patient's diaphragm EAdi may be measured as the patient's neural respiratory drive and may be multiplied by the amplification factor AF to describe the predicted ventilatory assist pressure P_(target).

An example of technique for measuring the electrical activity of the patient's diaphragm EAdi is described in U.S. Pat. No. 5,671,752 (Sinderby et al.) issued on Sep. 30, 1997. Although in the herein described embodiment the electrical activity of the patient's diaphragm EAdi is used as a representation of the patient's neural respiratory drive, it should be kept in mind that any other signal, for example the electrical activity of another respiratory related muscle can be used as the representation of the patient's neural respiratory drive.

The method and device for determining the level of ventilatory assist to be delivered to the patient uses a prediction (predicted ventilatory assist pressure P_(target)) of the pressure to be delivered to the patient by the mechanical ventilator in response to a given change of the amplification factor AF and a comparison between the predicted ventilatory assist pressure P_(target) and a resulting pressure P_(result) actually delivered to the patient by the mechanical ventilator in response to the given change of the amplification factor AF. For example, if the measured electrical activity of the patient's diaphragm EAdi is 10 μV and the amplification factor AF is increased from 0 cm H₂O/μV to 1 cm H₂O/μV, it can be expected that the predicted ventilatory assist pressure P_(target) will be 10 cm H₂O. If the resulting pressure P_(result) turns out to be 10 cm H₂O, this means that the patient has absorbed/accepted the whole ventilatory assist with no reduction of the patient's neural respiratory drive since the electrical activity of the patient's diaphragm EAdi remained unchanged. If, on the other hand, the electrical activity of the patient's diaphragm EAdi reduced from 10 μV to 1 μV, the resulting pressure P_(result) becomes 1 cm H₂O although the predicted ventilatory assist pressure P_(target) was 10 cm H₂O, showing that the patient does not require the increase of ventilatory assist and rather down regulates his/her neural respiratory drive (EAdi).

If the patient already receives ventilatory assist and, for example, the amplification factor AF is 1 and the electrical activity of the patient's diaphragm EAdi is 10 μV, it is possible to calculate an expected increase of the predicted pressure P_(target) if, for example, the amplification factor AF is increased to 1.5. The predicted ventilatory assist pressure will then be P_(target)=1.5 cm H₂O/μV×10 μV=15 cm H₂O. If the increase of the amplification factor AF from 1.0 cm H₂O/μV to 1.5 cm H₂O/μV increases the resulting pressure P_(result) from 10 cm H₂O to 15 cm H₂O, this indicates that the patient welcome the increase of ventilatory assist (increase in pressure) and maintains his/her neural respiratory drive (EAdi).

If, on the other hand, the amplification factor AF is increased from 1.0 cm H₂O/μV to 1.5 cm H₂O/μV but the resulting pressure P_(result) increases from 10 cm H₂O to 12 cm H₂O while the predicted ventilatory assist pressure P_(target) indicates a 5 cm H₂O increase, the patient does not welcome the whole increase in ventilatory assist and rather down regulates his/her neural respiratory drive (EAdi).

The method 100 and device 200 for determining the level of ventilatory assist to be delivered to the patient by the mechanical ventilator in response to the measure of a patient's neural respiratory drive multiplied by the amplification factor will now be described with reference to FIGS. 1 and 2.

Operation 101 (FIG. 1)

At a stable level of ventilatory assist, a calculator 201 (FIG. 2) calculates an existing predicted ventilatory assist pressure P_(target old) by multiplying the existing amplification factor AF_(old) by the existing electrical activity of the patient's diaphragm EAdi_(old), in accordance with the following relation: P _(target old)=AF_(old)·EAdi_(old)  (1)

Operation 102 (FIG. 1)

At a stable level of ventilatory assist, a pressure sensor 202 (FIG. 2) measures the existing resulting pressure P_(result old) delivered to the patient by the mechanical ventilator. This can be made, for example, by measuring the pressure in the mechanical ventilator or mechanical ventilator circuit.

When the neurally controlled ventilator system has been operated at a given amplification factor for a certain time, the existing predicted pressure P_(target old) and the existing resulting pressure P_(result old) will assume very similar if not the same values.

Operation 103 (FIG. 1)

A modifier 203 (FIG. 2) of amplification factor, for example a keyboard, is used by the medical personnel, for example a caregiver, to change (increase or decrease) an existing amplification factor AF_(old) to a new amplification factor AF_(new).

Alternatively, the modifier 203 can be implemented by an automatic computerized modifier requiring no intervention from the medical personnel.

Operation 104 (FIG. 1)

A calculator 204 (FIG. 2) calculates a new predicted ventilatory assist pressure P_(target new) by multiplying the new amplification factor AF_(new) by the existing electrical activity of the patient's diaphragm EAdi_(old), in accordance with the following relation: P _(target new)=AF_(new)·EAdi_(old)  (2)

Operation 105 (FIG. 1)

A comparator 205 (FIG. 2) compares the new predicted ventilatory assist pressure P_(target new) to the existing predicted ventilatory assist pressure P_(target old) using, for example, the following relation: ΔP _(target new-old) =P _(target new) −P _(target old)  (3) to show an anticipated change (increase or decrease) in pressure (ΔP_(target new-old)) that will be delivered to the patient by the mechanical ventilator.

Operation 106 (FIG. 1)

A pressure sensor 206 (FIG. 2) measures the new resulting pressure P_(result new) actually delivered to the patient by the mechanical ventilator following change from the existing amplification factor AF_(old) to the new amplification factor AF_(new). This can be made, for example, by measuring the pressure in the mechanical ventilator or mechanical ventilator circuit.

Operation 107 (FIG. 1)

A comparator 207 (FIG. 2) compares the new resulting pressure P_(result new) to the existing resulting pressure P_(result old) using, for example, the following relation: ΔP _(result new-old) =P _(result new) −P _(result old)  (4) to show an actual change (increase or decrease) in pressure (ΔP_(result new-old)) delivered to the patient by the mechanical ventilator.

Operation 108 (FIG. 1)

A comparator 208 (FIG. 2) compares the anticipated change (increase or decrease) in pressure ΔP_(target new-old) to the actual change (respectively increase or decrease) in pressure ΔP_(result new-old) using, for example, calculation of the following ratio: ΔP _(result new-old) /ΔP _(target new-old)  (5) to express a relation between the anticipated change and actual change in the level of pressure delivered to the patient by the mechanical ventilator. The ratio ΔP_(result new-old)/ΔP_(target new-old) will range between 0 and 1, such that the response to change (increase or decrease) of the amplification factor AF can be divided into classes as described hereinafter.

When the amplification factor is increased in operation 103 (FIG. 1), a ratio ΔP_(result new-old)/ΔP_(target new-old) equal to 1 suggests that the increase in ventilatory assist from the mechanical ventilator was welcomed by the patient, whereas a ratio ΔP_(result new-old)/ΔP_(target new-old) of 0.5 suggests that only 50% of the increase in ventilatory assist delivered by the mechanical ventilator was welcomed by the patient and that the patient down regulated his/her neural respiratory drive (EAdi).

Operation 109 (FIG. 1)

A calculator 209 (FIG. 2) includes a decision algorithm implemented to take a decision as to increase or decrease the amplification factor AF for example as described hereinafter.

When the amplification factor AF is increased in operation 103 (FIG. 1), the ratio ΔP_(result new-old)/ΔP_(target new-old) is positive. For example, the decision can then be as follows:

-   -   In response to a ratio         ΔP_(result new-old)/ΔP_(target new-old)=0.8 to 1, the decision         algorithm of the calculator 209 (FIG. 2) delivers a decision to         further increase the amplification factor AF;     -   In response to a ratio         ΔP_(result new-old)/ΔP_(target new-old)=0.5 to 0.8, the decision         algorithm of the calculator 209 (FIG. 2) delivers a decision to         maintain the amplification factor AF unchanged; and     -   In response to a ratio         ΔP_(result new-old)/ΔP_(target new-old.)<0.5, the decision         algorithm of the calculator 209 (FIG. 1) delivers a decision to         decrease the amplification factor AF.

When the amplification factor AF is decreased in operation 103 (FIG. 1), the ratio ΔP_(result new-old)/ΔP_(target new-old) is negative. For example, the decision can then be as follows:

-   -   In response to a ratio         ΔP_(result new-old)/ΔP_(target new-old)=−0.8 to −1, the decision         algorithm of the calculator 209 (FIG. 2) delivers a decision to         increase the amplification factor AF;     -   In response to a ratio         ΔP_(result new-old)/ΔP_(target new-old)=−0.5 to −0.8, the         decision algorithm of the calculator 209 (FIG. 2) delivers a         decision to maintain the amplification factor AF unchanged; and     -   In response to a ratio         ΔP_(result new-old)/ΔP_(target new-old)>−0.5, the decision         algorithm of the calculator 209 (FIG. 2) delivers a decision to         further decrease the amplification factor AF.

If the decision algorithm of the calculator 209 (FIG. 2) requests a change (increase or decrease) of the amplification factor, the complete, above described procedure including operations 101-109 is then repeated.

In operation 103 (FIG. 1), the modifier 203 (FIG. 2) of amplification factor will be used by the medical personnel to change (increase or decrease) or automatically changes (increase or decrease) the amplification factor AF as indicated. As a non limitative example, the modifier 203 (FIG. 2) may be designed to automatically change the amplification factor AF as a function of the ratio ΔP_(result new-old)/ΔP_(target new-old) according to a predetermined relation, for example by predetermined steps. Alternatively, it can be left to the medical personnel to decide to what extent the amplification factor should be changed through the modifier 203 (FIG. 2).

The calculator 201, calculator 204, comparator 205, comparator 207, comparator 208 and calculator 209 can be implemented by a computer or computers, for example a single, suitably programmed general purpose computer. 

What is claimed is:
 1. A method for determining a level of ventilatory assist to be delivered to a patient by a mechanical ventilator, comprising: measuring a patient's neural respiratory drive; using a first calculator to multiply the patient's neural respiratory drive by an existing value of an amplification factor to obtain an existing predicted ventilatory assist pressure; applying to the mechanical ventilator a first control signal representative of the existing predicted ventilatory assist pressure; measuring an existing resulting pressure delivered to the patient by the mechanical ventilator as a result of the first control signal; using a modifier to increase or decrease the amplification factor to obtain a new value of the amplification factor; using a second calculator to multiply the patient's neural respiratory drive by the new value of the amplification factor to obtain a new predicted ventilatory assist pressure; applying to the mechanical ventilator a second control signal representative of the new predicted ventilatory assist pressure; measuring a new resulting pressure delivered to the patient by the mechanical ventilator as a result of the second control signal; using a first comparator to compare the new predicted ventilatory assist pressure and the existing predicted ventilatory assist pressure to determine an anticipated change in pressure that will be delivered to the patient by the mechanical ventilator; using a second comparator to compare the new resulting pressure and the existing resulting pressure to determine an actual change in pressure delivered to the patient by the mechanical ventilator; using a third comparator to compare the anticipated change in pressure with the actual change in pressure; and using a third calculator to deliver a decision to increase, maintain or decrease the amplification factor in response to the comparison between the anticipated change and the actual change in pressure.
 2. A method for determining a level of ventilatory assist as defined in claim 1, wherein measuring the existing resulting pressure comprises measuring a pressure in patient's airways.
 3. A method for determining a level of ventilatory assist as defined in claim 1, wherein comparing the new predicted ventilatory assist pressure P_(target new) and the existing predicted ventilatory assist pressure P_(target old) comprises using the following relation: ΔP _(target new-old) =P _(target new) −P _(target old) to show an anticipated change in pressure ΔP_(target new-old) that will be delivered to the patient by the mechanical ventilator.
 4. A method for determining a level of ventilatory assist as defined in claim 3, wherein: comparing the new resulting pressure P_(result new) and the existing resulting pressure P_(result old) comprises using the following relation: ΔP _(result new-old) =P _(result new) −P _(result old) to show an actual change in pressure ΔP_(result new-old) delivered to the patient by the mechanical ventilator; and comparing the anticipated change in pressure ΔP_(target new-old) to the actual change in pressure ΔP_(result new-old) comprises calculating a ratio: ΔP _(result new-old) /ΔP _(target new-old) to express a relation between the anticipated change and actual change in the level of pressure delivered to the patient by the mechanical ventilator.
 5. A method for determining a level of ventilatory assist as defined in claim 4, wherein: the amplification factor is increased; the ratio ΔP_(result new-old)/ΔP_(target new-old) is positive; delivering a decision comprises: in response to the ratio ΔP_(result new-old)/ΔP_(target new-old) situated in an upper range, delivering a decision to further increase the amplification factor; in response to the ratio ΔP_(result new-old)/ΔP_(target new-old) situated in a middle range, delivering a decision to maintain the amplification factor unchanged; and in response to the ratio ΔP_(result new-old)/ΔP_(target new-old) situated in a lower range, delivering a decision to decrease the amplification factor.
 6. A method for determining a level of ventilatory assist as defined in claim 4, wherein: the amplification factor is decreased; the ratio ΔP_(result new-old)/ΔP_(target new-old) is negative; delivering a decision comprises: in response to a ratio ΔP_(result new-old)/ΔP_(target new-old) situated in a lower range, delivering a decision to increase the amplification factor; in response to a ratio ΔP_(result new-old)/ΔP_(target new-old) situated in a middle range, delivering a decision to maintain the amplification factor unchanged; and in response to a ratio ΔP_(result new-old)/ΔP_(target new-old) situated in an upper range, delivering a decision to further decrease the amplification factor.
 7. A method for determining a level of ventilatory assist as defined in claim 1, wherein measuring the new resulting pressure comprises measuring a pressure in patient's airways.
 8. A method for determining a level of ventilatory assist as defined in claim 1, wherein comparing the new resulting pressure P_(result new) and the existing resulting pressure P_(result old) comprises using the following relation: ΔP _(result new-old) =P _(result new) −P _(result old) to show an actual change in pressure ΔP_(result new-old) delivered to the patient by the mechanical ventilator.
 9. A method for determining a level of ventilatory assist as defined in claim 1, wherein the patient's neural respiratory drive is represented by the electrical activity of the patient's diaphragm.
 10. A device for determining a level of ventilatory assist to be delivered to a patient by a mechanical ventilator, comprising: means for measuring a patient's neural respiratory drive; a first calculator for multiplying the patient's neural respiratory drive by an existing value of an amplification factor to obtain an existing predicted ventilatory assist pressure used as a first control signal applied to the mechanical ventilator; a sensor of an existing resulting pressure delivered to the patient by the mechanical ventilator as a result of the first control signal; a modifier of the amplification factor configured to provide a new value of the amplification factor; a second calculator for multiplying the patient's neural respiratory drive by the new value of the amplification factor to obtain a new predicted ventilatory assist pressure used as a second control signal applied to the mechanical ventilator; a sensor of a new resulting pressure delivered to the patient by the mechanical ventilator as a result of the second control signal; a first comparator of the new predicted ventilatory assist pressure and the existing predicted ventilatory assist pressure to determine an anticipated change in pressure that will be delivered to the patient by the mechanical ventilator a second comparator of the new resulting pressure and the existing resulting pressure to determine an actual change in pressure delivered to the patient by the mechanical ventilator; a third comparator of the anticipated change in pressure and the actual change in pressure; and a third calculator of a decision to increase, maintain or decrease the amplification factor in response to the comparison between the anticipated change and the actual change in pressure.
 11. A device for determining a level of ventilatory assist as defined in claim 10, wherein the sensor of the existing resulting pressure comprises a sensor of a pressure in patient's airways.
 12. A device for determining a level of ventilatory assist as defined in claim 10, wherein the first comparator of the new predicted ventilatory assist pressure P_(target new) and the existing predicted ventilatory assist pressure P_(target old) uses the following relation: ΔP _(target new-old) =P _(target new) −P _(target old) to show an anticipated change in pressure ΔP_(target new-old) that will be delivered to the patient by the mechanical ventilator.
 13. A device for determining a level of ventilatory assist as defined in claim 12, wherein: the second comparator of the new resulting pressure P_(result new) and the existing resulting pressure P_(result old) uses the following relation: ΔP _(result new-old) =P _(result new) −P _(result old) to show an actual change in pressure ΔP_(result new-old) delivered to the patient by the mechanical ventilator; and the third comparator of the anticipated change in pressure ΔP_(target new-old) to the actual change in pressure ΔP_(result new-old) calculates a ratio: ΔP _(result new-old) /ΔP _(target new-old) to express a relation between the anticipated change and actual change in the level of pressure delivered to the patient by the mechanical ventilator.
 14. A device for determining a level of ventilatory assist as defined in claim 13, wherein: the modifier is configured to increase or decrease the amplification factor so that, when the amplification factor is increased, the ratio ΔP_(result new-old)/ΔP_(target new-old) is positive; and the third calculator is configured to deliver, when the ratio ΔP_(result new-old)/ΔP_(target new-old) is positive: a decision to further increase the amplification factor in response to the ratio ΔP_(result new-old)/ΔP_(target new-old) situated in an upper range; a decision to maintain the amplification factor unchanged in response to the ratio ΔP_(result new-old)/ΔP_(target new-old) situated in a middle range; and a decision to decrease the amplification factor in response to the ratio ΔP_(result new-old)/ΔP_(target new-old) situated in a lower range.
 15. A device for determining a level of ventilatory assist as defined in claim 13, wherein: the modifier is configured to increase or decrease the amplification factor so that, when the amplification factor is decreased, the ratio ΔP_(result new-old)/ΔP_(target new-old) is negative; and the third calculator is configured to deliver, when the ratio ΔP_(result new-old)/ΔP_(target new-old) is negative: a decision to increase the amplification factor in response to a ratio ΔP_(result new-old)/ΔP_(target new-old) situated in a lower range; a decision to maintain the amplification factor unchanged in response to a ratio ΔP_(result new-old)/ΔP_(target new-old) situated in a middle range; and a decision to further decrease the amplification factor in response to a ratio ΔP_(result new-old)/ΔP_(target new-old) situated in an upper range.
 16. A device for determining a level of ventilatory assist as defined in claim 10, wherein the sensor of the new resulting pressure comprises a sensor of a pressure in patient's airways.
 17. A device for determining a level of ventilatory assist as defined in claim 10, wherein the second comparator of the new resulting pressure P_(result new) and the existing resulting pressure P_(result old) uses the following relation: ΔP _(result new-old) =P _(result new) −P _(result old) to show an actual change in pressure ΔP_(result new-old) delivered to the patient by the mechanical ventilator.
 18. A device for determining a level of ventilatory assist as defined in claim 10, wherein the patient's neural respiratory drive is represented by the electrical activity of the patient's diaphragm. 